Extreme CPU usage while GPU rendering?
I'm certainly no expert, but I had a 95% CPU usage whilst rendering, even though I had chosen GPU Compute from the Render section.  What I didn't realise, was that I also need to select CUDA in User Preferences/System too.  After that I re-did the same Render and the CPU then hovered around 15% as expected.  V 2.79 if that makes any difference.  Will be interesting in the forthcoming version of Windows which supposedly will show the GPU usage in the Task Manager.